The present invention relates to an NR (Noise Reduction) system or circuit for use in audio equipment.
NR circuits are widely used for improving the S/N (Signal/Noise) ratio of an audio system such as a compact cassette deck. Recently, NR circuits have been applied to PCM (Pulse Code Modulation) systems employing a relatively small number of bits per sample, for instance, as commonly used in 8 mm video systems, where an excellent S/N ratio is not otherwise obtainable.
The NR circuit is not affected by bandwidth restrictions in the case of usage in an AM or FM system. Also, in a compact cassette deck, there is generally a sufficient bandwidth (0 to 20 KHz) available such that no adverse effects are produced.
Referring to FIG. 1, a prior art noise reduction system of a compact cassette deck is shown. For recording an input signal on a recording medium, low-level signals are emphasized by a compressor (encoder) 1 prior to recording. For playback, the signal reproduced from the recording medium is expanded by an expander (decoder) 3 to reduce the gain in those portions emphasized by the compressor 1 during recording.
Referring to FIG. 2, the NR circuit used in the compressor 1 and the expander 3 is shown. In this drawing, reference numeral 4 denotes a differential amplifier; 5, an emphasis circuit; 6, an amplifier having a controllable gain; 7, an emphasis circuit; 8, a weighting circuit; 9, a level detector; and 10, a compression/expansion switch. The switch 10 is set to the compression mode in the ON state and to the expansion mode in the OFF state. In the noise reduction system thus constructed, the level of the signal component of a given frequency applied to an input signal terminal 11 in FIG. 1 can be made to have the same level upon being outputted to an output signal terminal 12 because the bandwidth of the signal is not restricted by the transmission system, in this case, the recording medium 2.
This effect is illustrated in FIGS. 3A to 3C. FIGS. 3A to 3C show the voltage level of the signal component of the frequency f.sub.1 at respective points a, b, and c in FIG. 1. The signal component with a voltage level v as shown in FIG. 3A is increased to a voltage level u by the compressor by applying a certain gain before the signal is recorded on the recording medium 2. For playback, the reproduced signal, which has not been bandwidth limited by the recording medium 2, is applied to the expander 3. In the expander 3, the voltage level is reduced to a level w before being applied to the output signal terminal 12.
In the NR circuit shown in FIG. 2, the input to a level detector 9 during recording is the same as the input to the level detector 9 during reproduction if no bandwidth limiting occurs in the transmission system. Therefore the gain of the gain-controllable amplifier 6 is the same in both cases, as a result the input signal level v is the same as the output signal level w.
A PCM transmission system has a bandwidth of half the sampling frequency. For example, an 8 mm video system has an audio pass band below 15.734 KHz. Referring to FIG. 4A, a noise reduction system for use in a PCM transmission system is shown. In FIG. 4A, reference numeral 13 denotes a compressor and 14 denotes a PCM transmission system composed of a low-pass filter 15, a transmission converting system 16, and a low-pass filter 17. Reference numeral 18 denotes an expander. Referring to FIG. 4B, the construction of the transmission converting system 16 is shown. In this drawing, reference numeral 19 notes an A/D converter; 20, a digital recording system; 21, a magnetic recording and reproducing system; 22, a digital reproducing system; and 23, a D/A converter.
In the case of the PCM system 14 thus constructed, as shown in FIG. 4A, because the system has an upper limit of its audio pass band of f.sub.c, if the signal inputted from the input signal terminal 24 includes frequency components higher than the frequency f.sub.c, the signal levels of frequency components lower than f.sub.c are reduced below their original level when outputted to the output signal terminal 25. This action is shown in FIGS. 5A to 5D. FIGS. 5A to 5D show the voltage level of signal components of frequencies f.sub.1 and f.sub.2 at respective points a', b', c', and d' in FIG. 4A. Hereinafter, it is assumed that the frequency f.sub.c of the pass band of the PCM transmission system 14 and that the frequency f.sub.2 is higher than the frequency f.sub.c.
As shown in FIG. 5A, the signal components of the frequency f.sub.1 with the voltage level v.sub.1 and the signal component with frequency f.sub.2 with the voltage level v.sub.2 are increased to voltage levels u.sub.1 and u.sub.2, respectively, as shown in FIG. 5B, by the compressor 13 before being applied to the PCM transmission system 14. Since the PCM transmission system 14 has a pass band lower than the upper limit frequency f.sub.c, only the signal component of the frequency f.sub.1 is passed, as shown in FIG. 5C; the signal component of the frequency f.sub.2 is blocked.
Therefore, in the case where the NR circuit as shown in FIG. 2 is used in the compressor 13 and the expander 18 in FIG. 4A, the input to the level detector 9 during recording time is different from the input to the level detector 9 during reproduction. Thus, the gain of the amplifier 6 is also changed. Accordingly, the expander 18 reduces the voltage level of the component of the frequency f.sub.1 to the level w.sub.1 by the use of a gain larger than that applied by the compressor 13. Therefore, the value of v.sub.1 is v.sub.1 &gt;w.sub.1, which results in a disadvantage that the output level of the signal component of the frequency f.sub.1 is reduced in comparison with the input due to bandwidth restriction in the transmission system.